For a multitude of reasons, it is advantageous to use electric vehicles having rechargeable batteries rather than vehicles using internal combustion engines. Electric vehicles are inherently more efficient, meaning more energy is used in locomotion than lost to heat than in conventional engines. Also, electric vehicles do not exhaust any byproducts. However, the use of electric vehicles presents technical challenges. For example, the batteries in an electric vehicle must be recharged. Some electric vehicles are commercially targeted toward daily, low mileage use. Such vehicles are ideal for urban commuters. The batteries are chosen to provide a charge for approximately 50 miles before recharging is required. It is also well known that some batteries, for example lithium ion batteries, must be at temperatures above zero degrees centigrade in order to receive a charge. Furthermore, the chemical materials inside most batteries have resistive properties that are inversely proportional to temperature, meaning that as temperature increases, their internal resistance decreases and they are more readily, quickly, and efficiently charged. To that end, electric vehicles come equipped with some sort of apparatus for heating the battery before charging.
FIG. 1 shows a prior art charging system 100 for an electric vehicle. A battery 110 is capable of supplying sufficient voltage and current to power a motor 120, a cabin heater 130, and a DC/DC converter 140. All three of the motor 120, cabin heater 130, and DC/DC converter 140 are electrically coupled to the positive terminal of the battery 110 through the node A, and to the negative terminal of the battery 110 through node A′. In this example, the battery pack nominal voltage is 320V. There are two options to charge the battery 110. The first is an on-board charger 150, and the second is an off-board charger 160.
The on-board charger 150 is electrically coupled to the positive terminal of the battery 110 through node B and the negative terminal of the battery 110 through node B′. The on-board charger 150 is able to draw AC power from a wall plug and convert it into DC to charge the battery 110. The on-board charger 150 utilizes an AC/DC converter that is within the electric vehicle. Advantageously, all that is required is a power plug for connecting the vehicle to an appropriate power socket. As a result, it may be recharged most anywhere that a power socket is available. However, because the on-board charger 150 is constrained by the space available to it within the electric vehicle, the AC/DC converter is by extension also size limited. As a result, the amount of DC current the on-board charger 150 is able to generate is limited by size. So although convenient, it may take on the order of 5 hours to charge a battery pack. An off-board charger 160 is the second option. The off-board charger 160 is electrically coupled to the positive terminal of the battery 110 through the node C and the negative terminal of the battery 110 through the node C′. The off-board charger 160 is similar to the on-board charger 150 in that it comprises an AC/DC converter for converting an AC voltage from a wall to a DC voltage appropriate to charge the battery 110, in this example 320V. However, because the off-board charger 160 is not housed within the electric vehicle, it is not constrained by size. Therefore, the AC/DC converter therein is able to be larger and more robust than the AC/DC converter of the on-board charger 150. As a result, the off-board charger 160 is able to generate far more current, and charge the battery 320 much faster, on the order of an hour.
Nodes A, B and C are electrically coupled to the positive terminal of the battery 110 via switches 101, 102 and 103 respectively. The negative terminal of the battery 110 is electrically coupled to the nodes A′, B′ and C′ through the switch 105. The switches 101-105 are preferably analog switches, such as contactors, relays or transistor devices, including bipolar, MOSFET, or IGBT implementations. In a charge condition, one of the switches 102 or 103 is closed to electrically couple one of the on-board charger 150 and off-board charger 160 respectively to the positive terminal of the battery 110. Switch 105 is closed as well. As a result, a charging loop is formed through B to B′ or C to C′. However, as mentioned above, a battery must be above zero degrees C. in order to properly charge. Therefore, especially in cold weather climates, it is advantageous to have a battery pre heater 170. In this prior art, the battery pre heater 170 is electrically coupled to the DC/DC converter 140, and the switch 101 must be closed to form a path to power the DC/DC converter 140. Some battery pre heaters 170 work off of 12V DC. However, one of the switches 102, 103 must be closed depending on whether on-board or off-board charging is being utilized. Also, the switches 101 and 105 must be closed in order to form a closed circuit. Therefore, one of the loops B-B′ or C-C′ along with A-A′ which is highly undesirable, since the battery 110 may be damaged beyond utility, or it may explode causing severe injury to a person that may be near it. What is needed is an electric vehicle battery pre-heating system wherein the heater is de-coupled from the charging apparatus during a pre-heating process.